With regard to personnel, system and fire safety in electric power supply systems and equipment, the monitoring of the insulation resistance, i.e. the resistance in the power supply system to be monitored including the resistances of all equipment connected thereto against ground, is an indispensible prerequisite for a failure-free operation of the electric system. Without a sufficiently high insulation resistance, protection against direct and indirect contact is no longer ensured. Malfunctions of the electric systems may cause danger to personnel, lead to loss of production or to a halt of the system, or short-circuit currents or ground fault currents can cause fires and explosions. In newly assembled systems and equipment, the insulation resistance is usually high enough, but over the course of operation of the system it can be decreased by electrical and mechanical influences and by environmental influences and the effects of age.
A measurement of the insulation resistance can be carried out in grounded networks (TN systems) as well as in ungrounded networks (IT systems), the constant monitoring of the absolute insulation value during operation playing a prominent role. For the measurement, independently from the type of network, a signal generator and a measuring device are interposed between the active conductors of a power supply system and ground, the signal generator impressing a measuring signal voltage into the network so that in the case of an insulation fault a closed circuit is formed in which a measuring current proportional to the insulation fault sets in. Said measuring current is registered by the measuring device and thus allows a statement concerning the strength of the insulation resistance.
In today's modern networks, a plurality of the equipment is provided with electronic components. In order to prevent measurement distortions, caused for example by direct current components generated by inverters, the measuring methods have been constantly developed further in view of a reliable insulation monitoring. Hence, it is known practice to use the method of superimposing a DC measuring voltage in pure alternating current networks without distorting direct current components, whereas in faulty environments a controlled, specifically clocked measuring voltage with square-shaped generator pulse forms is employed. Square pulse sequences of this sort used in monitoring devices currently on the market prove to be relatively insensitive to broad-band interference signals, but they show an insufficient robustness in case of narrow-band interferences.
From DE 38 82 833 T2, it is further known to supply an AC voltage reference signal on the basis of which the real and imaginary part of a complex insulation impedance can be calculated via a suitable evaluation circuit. Since the sinusoidal reference signal has only one frequency component, it is relatively sensitive to interference signals that occur in the range of the frequency of the reference signal.
In order to be able to better deal with low-frequency interferences, in EP 0 593 007 B1, the calculation of the complex-valued network leakage impedance on the basis of two periodic measuring signal alternating voltages with constant amplitudes is proposed. However, the frequencies of the two alternating voltages should lie in a range largely free of interference signals and be relatively low for the network leakage values to be calculable, neglecting the network inductivities, by a simplified function correlation.
The methods known from the state of the art for determining the insulation resistance provide insufficient interference resistance against narrow-band interference signals, in particular against interference signals with only one discrete frequency. Moreover, the measuring signals used are tied to evaluation methods that are adjusted to the respective measuring signal form and show different efficiencies with regard to the suppression of interference signals. All in all, the problem of interference suppression has been unsatisfactorily solved thus far.